Tandem switching system



E95@ R. C. AVERY TANDEM SWITCHING SYSTEM 4 Sheets-Sheet l Filed 00in. 9,1947 /NVENTOR R. c AVERY A7' TORNE V De@ M9 H95@ R. c. AVERY 533,342

TANDEM SWITCHING SYSTM Filed oct. 9, 194'? 4 sheets-sheet 2 POLAR/Z50POLAR/Z50 /A/VE/vrof? R. C AVERY @a 3129 1195 R. Q. AVEY 25339342TANDEMNSWHCHING SYSTEM 'v Filed QC. 9, 1947 4 Sheets-Shee C. VERV er y*R. c. AVERY 533,342

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TANDEM SWITCHING SYSTEM @HV vg@ Dec. E29 E95@ Fuec oct. 9, 1947' w @PmPatented ec. l2, 1950 TANDEM SWITCHIN G vSYS TEM Robert Campbell lAvery,Jackson Heights, N. Y., assignor to American Telephone and TelegraphCompany, a corporation of New York Application (lctober 9, 1947, SerialNo. 778,977

4 Claims.

This invention relates to telephone systems and has for its object tofacilitate the 'handling of tandem connect-ions in such systems.

Heretofore, telephone `calls between yoffices having no directconnecting trunks have, in general, been handled either -by specialtandem equipment or by the use of a `discriminating digit which -resultsin a reduction lof the Iamount of equipment available for localLserv-ice.

In accordance with the present invention, means is provided for handlingtandem calls ata local oce without sacriiicing any of the localequipment.

The invention is Vespecially applicable lto lcross bar systems of thetype disclosed in Uni-ted States Patents 2,089,921 to W. W. Carpenter,August i0, 1937; 2,235,803, to W. W. Carpenter, March l8r, 1941; and2,232,3711to Deh-n and Myers, February 18, 1941.

Accordingly means is provided in the vcom-mon control equipment of sucha 4sys-tem for registering either a line designation or a trunkdesignation and inthe latter Icase for modifying the operation -of thecontrol equipment to select ani'd-le trunk.

More specically, the Aterminati-ng sende-r which normally responds tothe reception of a live-digit line designation to associate itself witha switch controlling marker, res-ponds to a two-digit 'trunk designationto associate itself with a marker and to render the marker-effective toV.handle the twodigit trunk designation.

These and other features Lof the invention 'will be more apparent from`a consideration of the following description read lin connection withthe attached drawing in which:

Figure `1 shows a tandem trunk, together with -a schematic showing of asender link circuit and yoi a repeater link circuit;

Fig. '2 shows a rever'tive pulse repeater;

Fig. 3 shows partsof a vterminati-ng sender;

Fig. 4 shows a schematic marker-connector and parts of a terminatingmarker; and

1Fig. 5 shows the manner vin which `Figs. 1 to 4 are to be arranged.

As described in detail in the above-mentioned patents the setting up ofa telephone connection in the cross bar system takes place in twostages, an originating stage and a terminating stage. The originatingstage includes the automatic eX- tension of the calling line to adistrict junctor, followed by a directive operation in which thedesignation of the wanted office and line is registered in anoriginating register sender, and the sender vis associated with anoriginating marker whichcontrols the selection of a trunk in accord--ance with the designation of the wanted `office as received -from thesender and establishes .a connection 'between the trunk vand theydistrict junctor. 'The terminating stage includes the .automaticselection of a terminating vsender .in response to the seizure of theincoming junctor in `which the selected trunk terminates. When theterminating sender has been selected, it is `connected with the:originating 4sender 4and the number of the wanted line is transmittedfrom the `originating .sender to the terminating sender hy means of live.series of revertir/'e pulses, after which the originating sender isreleased. A .terminating marker is then associated with the terminatingsender, receives the .line ,number from the sender., .deter-mines theiocatio-n :and :class of the Awanted line, tests it to determine whetherit is idle .and `establishes -a Vconnection between the incoming junctorand the called line, thus Accmpleting the connection between :the:calling Iand .the called line.

In Carpenter Patent 2,235,803 itis pointed out( that the five trainszo'f reyertive Epulses may be used for the vcontrol of panel typeselectors .in `a ypanel terminating cnice. Because :of the limitations4imposed by the .panel selectors., the v.first Iset .of freverltiveimpulses transmitted :never exceeds live, while Vthe second set ofrevertiue impulses never exceeds Tour.

The originating .cross fbar sender lis :also equipped -to receive twosets yof revertive impulses to `control an intermediate panel fofceselector or a cross lbar tandem sender when required. The number ofthesepulses is determined .by the marker .in `'accordance with lthe routezand, therefore, any arbitrary `number of pulses may be used.. The use`made lof this -fact in the presen-t disclosure will be pointed vouthereinafter.

The drawings of the present disclosure are based on the drawings of the.above-mentioned Carpenter Patent .2,099,921 to lwhich reference is madefor -a description of the complete `circuits and aoperations. Forconvenience in :such reference, the :circuit A'elements xtaken from the.Carpenter disclosure, which are to :be .ioundin Figs. 3 'and 4, havebeen :given the ,same reference numerals as used in .that patent, theadded .elements being identified by letter designations.

When the equipment at the nrigIinati-ng o'lce has functioned to :selectthe trunk of Fig. 1, fa -circuit is completed from battery through theupper wjindingIof -relay A, `contact f'l -of relay T, contact 4 of relayD, -over 1'the tip conductorT and thence as shown Iin yCarpenter Patent2,245,803, through the winding of a stepping relay in the originatingsender, back to ring conductor R, contact i3 of relay D, contact 6 ofrelay T, contact I3 of relay D to ground. Battery through resistance AIis connected in parallel with the upper winding of relay A over ContactE of relay D to improve the operating condition for the sender relay,but relay A is able to operate in the circuit traced.

Relay A, in operating closes a circuit from ground at its front contact,contact 8 of relay D, Contact i of relay RST to the start conductor SSTfor the sender link circuit. Thesender link control circuit operates asdescribedin Patent 2,089,92i to close cross-points in primary andsecondary link switches to extend the six conductors T, R, D, CO, FC andSS through to conductors 3726, 3725, 3722, 3724, 3723 and 5|46respectively in an idle sender such as shown in Fig. 3. When the senderis ready to receive the line designation, relays 3700 and 3701i and 37Mare operated. Relay 3730, in operating, extends tip conductor 3720 overits contact l, contact of relay TAN, contact l of relay 370| to thewinding of relay 30|5 and battery. Relay 3706 also extends ringconductor 3723 over its contact 2, contact 6 of relay TAN, contact 4 ofrelay 333| to the winding of relay 3030 and ground. Relay 30|5 and thestepping relay of the originating 'sender operate in this circuit, butrelay 3030 is shunted by the ground at contact IE) of relay D.Responsive to the operation of relay 30|5, a circuit is closed fromground over the iront contact of relay 3704, contact of relay MRL.contact 3 of relay 3700, conductor 3724, through the sender link toconductor CO, contact 9 of relay H, contact 2 relay D to the winding ofrelay T and battery. Relay T operates in this circuit, disconnecting thewinding of relay A from the tip and ring conductors, as well asdisconnecting ground from the ring conductor, thereby leaving relays30|5 and 3000 connected to the stepping relay of the originating senderand permitting relay 3000 to operate in the pulsing circuit. Relays3000, 30-|5 and the stepping relay in the originating sender function asset forth in the Carpenter Patent 2,089,921 to bring about thegeneration, transmission and registering of trains of revertive pulsesto cause the setting of the cross bar register. Only the incoming brushvertical and the incoming group vertical of the register are shown.

When relay 30l5 operates in the pulsing circuit above traced, it in turnoperates relay 3706. Relay 3352 is operated and released alternately asa part of the setting of the cross bar register, being operated afterframe registration, which takes place prior to the establishment of thepulsing circuit, and released after the registration of the first trainof revertive pulses on the ncoming brush vertical. of pulses, thepulsing circuit is opened by the originating sender long enough for bothrelay 30|5 and relay 3705 t0 release. Therefore, at the beginning of thesecond train of pulses, a circuit is closed from battery throughresistance LOCI, winding of relay LOC, front contact of relay '3703,back contact of relay 30|2 to ground at contact 2 of relay 37|1l. RelayLOC operates and locks over its contact 2 to ground at contact 2 ofrelay 37|4.

When relay 3703 releases at the termination of the second train ofpulses, registered on the incoming group vertical, a circuit is closedfrom battery through resistance TAN I, winding of relay TAN, contact ofrelay LOC, back Contact of At the end of each train QT' relays 3703` and30|2 to ground at contact 2 of relay 37|4.

In the case of a local call, as mentioned above,

one of the lower four cross-points, for example cross-point 2933 wouldbe closed. Therefore a circuit would be completed from ground at contact2 of relay 37 I4, outer right contact of crosspoint 2933, to batterythrough resistance TAN in shunt of the winding of relay TAN, eitherpreventing the operation of that relay or causing it to releaseimmediately.

. With relay TAN released, the pulsing circuit is reclosed to theoriginating sender and the next set of revertive pulses may be sent.When five sets of revertive pulses have been transmitted and registered,relay 370| is operated to transmit a reverse battery signal to theoriginating sender, after which relay 3709 is operated. With relay 3739operated, battery is connected over the back contact of relay 3020,front contact of relay 3709, contact l of relay 37M to the startconductor 373| leading to the terminating marker connector, to bringabout the connection of the sender with an idle terminating marker suchas shown in Fig. 4.

On the other hand, in the case of a tandem call, the originating senderwill be set to receive two sets of pulses identiiied as ofce brush andoce group selection pulses under the control of registrations set up bythe originating marker in accordance with the route to be used. If thecall is to be routed through a combination oice of the sort shown in thepresent disclosure, the oiice group register will be set to require anumber of pulses from six to nine to satisfy the originating sender, forexample 8. Therefore, crosspoints in the incoming brush and incominggroup verticals in the terminating sender of Fig. 3 will be closed asdetermined by the oice brush and oice group registers of the originatingsender. Assuming that eight revertive pulses are required by the officegroup register, cross-point IGS in the incoming group vertical will beclosed. With cross-point IG8 operated, a circuit is completed fromground at contact 2 of relay 37M, outer right contact of cross-pointIG8, to battery through resistance LOCI, in shunt of the winding ofrelay LOC, causing relay LOC to release and leaving relay TAN operated.

Relay TAN, in operating, closes a locking circuit for itself over itscontact 5 to ground at contact 2 of relay 37| Il. At contacts and 6 ofrelay TAN, the pulsing circuit is opened in the terminating sender toprevent the advance of the originating sender to position to make trunktest. At contact Il, relay TAN closes the start circuit for the markerconnector, the circuit extending in this case from battery over the backContact of relay 3020, contact 4 of relay MRL, contact 3 of relay LOC,contact 1l of relay TAN, contact of relay 37H1 to conductor 373|. When amarker has been associated with the sender, relay TAN, at contact 9,connects ground to conductor TAN@ which is connected through the markerconnector and over conductor TAN4 to the winding of relay TAN! to informthe marker that it is to handle a tandem call.

When the rnorifer and sender have been com gona cese, and 33|respectively, while sixteen registration transfer paths are set upbetween the cross bar register of the sender and the relay 5. registersof the marker. As desscribed in detail in Carpenter Patent 2,089,921,ground is' connected according to a predetermined code to certaincontacts of the Various cross-points, thereby operating` thecorresponding marker register relays directly.V The remaining contactsare connected to checking conductors such as conductors and 556i whichare supplied with ground from the marker, whereby, in the case of alocal call, all of the register Vrelays are operated, completing a chaincircuit over all oi' the register relays. Included in this chain circuitare contacts on checking relays, such as relay 682i, which are operatedfrom the checking ground supply, and on reorder relay 1232i which A l isoperated .from ground on the checking conductor over contact i oi relayTAN, conductors R03 and iitei and contact le of relay TANI. This chaincircuit extends from battery through the lower winding of relay e826,contacts oi the checking relays including relay M22, contact of reorderrelay iSZl, contacts of the units register relays (not shown), contactsof the tens register relays including relay fl, contacts of the hundredsregister relays 923, 6922, i922 and 59%, contacts of the thousandsregister relays (not shown) contacts of the frame register relaysincluding relay Sill to ground through the left winding ci relay 3325.With relays and 'i825 operated and locked, the checking ground isremoved and only those relays operated directly from the sender remainoperated.

From the thousands register, in combination with relay i325 of thehundreds register, one oi the ve hundreds relays, such as relays ESM andiifiA, is operated, which in combination with a hundred-block relay suchas relay HNO oper-- ated from the hundreds register and a twentyblockrelay such as relay 62de operated from the tens register in combinationwith a units register relay causes the selection of a line choice and agroup of twenty lines to be tested.

When the marker has seized the line choice, ground is connected toconductor 'i connected through the marker connector to conductor 3'1'23and through the sender link to conductor FC, completing a circuitthrough the winding of relay F to battery. Relay F at contact l groundscon ductor LC, thereby operating, in cooperation with the marker, theincoming link connector to associate the marker with the incoming linkswitches.

Having found the called line idle, an idle channel is selected, theproper ringing condition is set up in the trunk and the continuity oithe prepared connection is tested. The latter test causes theoperationof relay fitti?, which, if the ringing control has been operatedproperly, causes the operation oi relay lie. With relay tilde operated,ground is removed from conductor 556s releasing relay F in the trunkwhich disconnects conductor S from conductor SL as a signal to themarker. In response to this signal, ground is connected to releaseconductor ElZ.

With relay TAN normal in connection with a local call, ground onconductor i, extending to conductor .liet completes a circuit overcontact oi relay TAN, back contact of relay Sl'l to battery through thewinding oi relay 37S? causing the sender to release in the usual manner,

In the case of a tandem call, since cross-points are closed only in theframe Vertical (not shown) and in the incoming brush and incoming groupverticals of the sender, only the relays of the frame register and thethousands register and relay @24 of the hundreds register can beoperated in the marker as a result of the checking operation. Inaddition relay in the sender opens the circuit of reorder relay lll,preventing its operation. However, since relay TANI is operated, abridge is closed around the contacts of thev hundreds, tens and unitsregister relays, and of reorder relay A82?, the chain checking circuitin the case of a tandem call extending from battery through the lowerwinding of relay M265, contact or relay a'ii, contact 5 or relay TANI,contact of relay 39251, over the contacts of the thousands registerrelays and of the frame regisn ter relays, including relay wel to groundthrough the left Winding of relay @3525.

The operation oi a five hundreds relay such as relay @Se or tfiA takesplace as for a local call. The line choice start lead is closed throughcontact 2 of relay TANS and contact i of the operated five hundredsrelay. Since relays 5921,

and edit are not operated, the Zero hundred-block relay HNO wouldnormally be operated, and similarly, since none ofthe tens registerrelays are operated the zero twenty-block relay 52de would normally beoperated. However, with relay TAN! operated, the circuit for operatingthe HNO relay is opened at contact i of relay TANi, and a circuit foroperating the hundred-bloclr relay for the tandem trunks is closed atcontact i of relay TAN l, While the circuit for operating a twenty-blockrelay is transferred over contact i of relay TAN! to contacts of thefive hundreds relays to cause the operation of the twenty-block relayassociated with the tandem trunks. These trunks are located on the linelink frame are tested and an idle one selected in the manner describedin Carpenter Patent 2,089,921 for selecting an idle trunk leading to aprivate branch exchange.

In general the selection of an idle channel and the establishment of theconnection between the trunk of Fig. 1 and the selected trunk take placein the usual manner. However, since the ringing equipment is located atthe terminating onice, the battery for the ringing control relays issupplied oVer contacts of relay BS so that, with relay TAN! operated,relay BS is operated from ground over contact 8 of relay TAN I and noneof the ringing control relays may be operated.

Relay TANE also closes a circuit at its contact 1 for operating relay@Mii to cancel the continuity test as is done in the case of interceptedcalls. After an interval following the operation of the incoming andline switches, relay m38 is operated under the control of relay Milli,in turn operating relay 4009 over contact il of relay TAN l independentof the ringing control relays. Relay 1369 causes the release of relay Fand the grounding of conductor ll and therefore of conductor 5658. Onthe sender, since relay TAN is operated, release conductor 5568 extendsover contact 'l of relay TAN to the winding of relay MRL and battery.Relay MRL locks over its contact 3, contact Il of relay TAN, to groundat contact 2 of relay 31M, At contact ll, relay MRL disconnects batteryfrom conductor 313i, thereby releasing the marker connector and themarker. At contact l, relay MRL disconnects ground from conductor 3'l24,releasing relay T in the trunk circuit.

With relay T released, relay A is again connected to the incoming trunkconductors T and R and reoperates.

When relay F was operated, as previously described, it closed a circuitfrom ground over its contact I to the winding of relay RST and battery.Relay RST operated, locking over its contact 3 to ground supplied toconductor SS from conductor MB in the sender. Therefore, when relay Areoperates, it closes a circuit from ground over its front contact,contact 8 of relay D, contact 2 of relay RST to conductor RST causingthe repeater link circuit to operate and connect the incoming andoutgoing trunk conductors to an idle revertive pulse repeater such asshown in Fig. 2. This -connection is established and held under thecontrol of a hold magnet, for example, magnet HMS.

The reconnection of relay A to the incoming trunk conductors T and Rcauses the originating sender to complete trunk test and connect thestepping relay to the trunk in readiness for receiving the iirst set ofrevertive pulses for transferring the line number.

With conductors T and R connected to conductors T2 and R2 through therepeater link, relay L2 of the revertive pulse repeater operates in acircuit for battery through its winding, contact I of relay PR, contactl of relay FC, contact 3 of relay RVB, contacts 4 and E of relay RVI,conductors T2 and T, through the stepping relay of the originatingsender, back over conductor R to ground over contact I3 of relay D,contact 5 of relay T and contact I of relay D.

Relay L2 in operating closes an obvious circuit for relay LIZ, which inturn operates relay ON. Relay ON locks over contact 3, normal contact ofrelay TC, and conductor G to ground at contact 2 of the hold magnet HMQand at contact 5 exten-:ls this locking ground over conductor H andcontact I of hold magnet HMS to the Winding of that magnet to hold thelink circuit, independent of the start circuit. At contact 2 relay ONcloses a circuit which extends from ground at that contact over contactI of relay RVS through the winding of relay RV to battery. Relay RV!!operates and closes a circuit from battery through resistance RV,winding of relay RVS, contact 3 of relay RVi.l to ground at contact 2 ofrelay ON. Relay RV operates, locking over its contact 3 to ground atcontact 2 of relay ON, independent of relay RV4.

With relays RVli and RV5 operated a circuit is closed from ground atcontact 2 of relay ON, contact 4 of relay RVS, contact i oi relay RVA,conductor CO2, connected through the repeater link to conductor COI,lower winding of relay H to battery. Relay H operates in this circuit,extending its operating ground over its contact 5, Contact 2 of relay Dto the winding of relay T, which operates, and again disconnects relay Afrom the incoming trunk, leaving the originating sender connected withthe repeater of Fig. 2.

At contact 6 relay H connects ground through resistance SI, contact 6 ofrelay F to conductor S to hold the switches connecting the trunk of Fig.l with the incoming trunk at the terminating office. At contact 3, relayH connects ground to conductor CO completing a circuit extending throughthe sender link to conductor 3126, contact 3 of relay 3WD, contact 2 oirelay MRL, back contact of relay S'Il to battery through the winding ofthe sender release relay 3797, causing the sender to release, in turnreleasing the sender link.

When relay T operates, it disconnects ground from conductor R, whichextends through the repeater link to conductor R2, contact 2 of relayRV2 to the Winding of relay STPI and ground.

Relay STPI operates in the incoming pulse circuit in series with relayL2 and the stepping relay of the sender, operating relay GR, which inturn operates relay FC. Relay FC locks over the back contact of relay FOand contact I of relay FC to ground at contact 2 of relay ON. With relayFC Operated an outgoing pulsing circuit is closed from conductor TI,contact 2 of relay F, conductor FTI, through the repeater link toconductor FIZ, contact 2 of relay FC', back contact of relay BO,windings of relays STP and OF, conductor FRZ through the repeater linkto conductor FRI, Contact 8 of relay F to conductor RI. These conductorsextend to the terminating once where revertive pulses are generated inthe setting of panel switches or by a terminating cross bar sender inaccordance with the nature of that terminating oce.

For a description of the detailed operation of the revertive pulserepeater of Fig. 2 in repeating pulses between the terminating oice andthe originating sender, reference is made to applicants Patent No.2,383,759, granted August 28, 1945.

When revertive pulsing is completed, a reverse battery pulse from theterminating oice operates relay OF which operates relay RV I. Relay RVIlocks over its contact 2 to ground at contact 2 or" relay ON and in turnoperates relay RV2. At the end of the reverse battery pulse, relay OFreleases closing a circuit from ground at contact 2 of relay ON, normalContact of relay OF, contact .5 of relay RV2 to battery through thewinding of relay RV3.

Relay RVS operates and at its contact I opens the circuit of relay RV2.Relay RVll releases and disconnects ground from conductor CO2, releasingrelay T, to reconnect relay A to the incoming conductors T and R. Thecircuit of relay H is also opened but, since relay H is slow to releaseit does not release at this time. With relay T released, ground isconnected over Contact I0 of relay D, contact 6 of relay T, contact I3of relay D, conductor R through the repeater link to conductor R2,Contact 3 of relay RVi, contact 2 of relay RV2, contact 2 of relay RVEto battery through resistance RV in shunt of the winding of relay RVS,causing relay RVS to release. The release oi relay RV5 closes a circuitfrom ground over contact 5 of relay RV3, contact 5 of relay RV E,resistance A, winding of relay TC, conductor D2, through the repeaterlink to conductor D, contact i and upper Winding of relay H to batterythrough resistance H to hold relay H operated. The circuit also extendsfrom conductor D through the winding of relay D to battery.

Relay D operates in this circuit, but polarized relay TC is energized inthe non-operate direction and continues to hold its right contactclosed. Relay D in operating, connects ground at its contact 9 toresistance SI and thence to conductor S to provide an additional holdingcircuit for the incoming and line link switches. .At contacts l and i3it opens the previously traced circuit for relay A and at contacts 5 andI2 closes a new circuit for that relay including the left windings ofrepeating coil R.

When trunk closure occurs at the originating oice relay A operates and,with relay D operated, completes a circuit from ground over the contactof relay A, contact l of relay D, contact 2 of relay H, lower winding ofrelay DI to battery. Relay DI operates in this circuit, locking in acircuit from battery through its upper winding and contact 2 to groundat contact I of relay D. Re-

lay DI connects the outgoing tip and ring conductors Ti and Ri, overcontacts 2 and 8 of relay F, contacts I and 3 of relay Di to the rightwindings of repeating coil R and the winding of polarized supervisoryrelay S, thereby establishing a trunk closure toward the terminatingoffice. Relay Di also closes a locking circuit for relay D under thecontrol of relay A which may be traced from ground at the contact ofrelay A, contact 'l of relay D, contact 5 oi relay Dl, to batterythrough the winding of relay D. At its contact 4, relay Di extends abranch from the locking circuit of relay D, over contact 3 of relay H tobattery through resistance H, in shunt of the locking winding of relayH, causing relay H to release. The locking ground of relay D extendsback over the operating circuit of relay D to conductor D, through therepeater link to conductor D2, winding of relay TC to battery throughresistance TC. The valuesv of resistances A and TC are such that relayTC may now operate, opening the locking circuit of relay ON, causingthat relay to release and disconnect ground at its contact 2 from theholding circuits of the various relays in the revertive pulse repeater.tion of relay TC also opens the holding circuit for hold magnetrestoring the repeater link to normal. Relay TC is held operated overits left contact to ground on conductor G until magnet HMB releases.

With relay D operated, the circuit of relay T is extended over Contact tof relay D to the front contact of supervisory relay S and now serves t0reverse the connection of battery and ground through the windings oirelay A to the incoming conductors T and R in response to supervisorysignals from the terminating oice. The connection is .held under thecontrol of the calling subscriber.

In the sender, if relay 392e is operated while handling a tandem call,for example because of trouble at the originating sender, battery isremoved from conductor li to cause the immediate release of the markerand ground is connected to resistance TANi to release relay TAN andpermit the sender to complete its function as for a local call.

In the marker, while handling a tandem call, if no idle trunk can hefound, relay 33638 is operated, connecting ground over contact s ofrelay TANI, conductor 558e through the marker connector to conductorR03, contact 3 or" relay TAN to the winding of relay 3522i?, whichfunctions as above-described to release the marker and restore thesender to the local call condition.

To guard against false tandem indication to the marker by a false groundon conductors TAN3, TAN'i or the linkage in the marker connector, relayCK is connected to conductor TANS over contact iii of relay TAN whenrelay TAN is not operated. Ii relay CK is operated, it extends itsoperating ground to conductor 565s to prevent the normal release of theregister relays and cause the marker to time out and make a troublerecord.

While the trunk circuit shown in Fig. 1 is especially adapted forhandling tandem calls, the invention is not limited to the use ofseparate trunks for local and tandem calls but may be used with trunksincorporating the features of both local and tandem trunks.

What is claimed is:

l. In a telephone system, an cnice, trunks incoming to said oiiice,subscribers lines and interoice trunks outgoirrT from said oice,register senders in said office, connecting means, means The operain one4of ysaid trunks to operate said .connecting means to select one of saidsenders and to connect said selected sender with said one trunk, aplurality of registers in said sender, means operative over said onetrunk to successively operate all of said registers to register thedesignation of a wanted line thereon, a plurality of switch controllingmarkers, means normally dependent on the operation of all of saidregisters for connecting a switch controlling marker with said sender,means to render said marker connecting means responsive to the operationof a predetermined lesser number of said registers, a register checking.circuit in said marker normally completed under the control of all ofsaid registers, means normally .under the control of said checkingcircuit to cause said marker to control the extension of said incomingtrunk to said wanted line, means to complete an alternative registerchecking circuit under the control of said predetermined lesser numberof said registers and means under the control of said alternativeregister checking circuit to cause said marker to control the extensionoi said incoming trunk to one of said outgoing trunks.

2. In a telephone system, an office, trunks incoming to said omce,subscribers lines and interoice trunks outgoing from said oiiice,register senders in said oflice, connecting means, means in one of saidtrunks to operate said connecting means to select one of said sendersand to connect said selected sender with said one trunk, a plurality ofregisters in. said sender, means operative over said one trunk tooperate all or said registers to register the designation of a wantedline thereon, a plurality of switch controlling markers, markerconnecting means, means normally dependent on the operation or all ofsaid registers to operate said marker connecting means and to releasesaid sender, a local call relay and a tandem call relay in said sender,means under the control of said register operating means to successivelyoperate said relays following the operation of each of the first two ofsaid sender registers, means operated in accordance with theregistration set up on said second register to selectively release oneof said relays, means under the control of said local call relay whileoperated to render said tandem call relay ineffective and means underthe control of said tandem call relay, following the release oi saidlocal call relay to operate said marker connecting means with aplurality of said registers unoperated.

3. In a telephone system, an oilice, trunks incoming to said oiiice,subscribers lines and interoiilce trunks outgoing from said office,register senders in said office, connecting means, means in one of saidtrunks to operate said connecting means to select one oi said sendersand to connect said selected sender with said one trunk, a plurality ofregisters in said sender, means operative over said trunk to operate allof said registers to register the designation of a wanted line thereon,a plurality of switch controlling markers, marker connecting means,means normally dependent on the operation of all of said registers tooperate said marker connecting means and to release said sender, a localcall relay and a tandem call relay in said sender, means under thecontrol o1" said register operating means to successively operate saidrelays following the operation oi each of the iirst two of saidregisters, means operated in accordance with the registration set up onsaid second register to selectively release one of said relays, meansunder the control of said local call l 1 relay While operated to rendersaid tandem call relay ineffective, means under the control of saidtandem call relay, following the release of said local call relay tooperate said marker connecting means with a plurality of said registersunoperated, and means under the control of said tandem call relay torender the connected marker effective to control the connection of saidincoming trunk to an outgoing trunk.

4. In a telephone system, an ofce, trunks incoming to said ofce,subscribers lines and interoce trunks outgoing from said olce, registersenders in said oce, connecting means, means in one of said trunks tooperate said connecting means to select one of said senders and toconnect said selected sender With said one trunk, a, plurality ofregisters in said sender, means operative over said trunk to operatesaid registers, means normally dependent on the operation of all 0f saidregisters to register the designation of a wanted line for connecting aswitch controlling marker with said sender, a plurality of registers insaid marker operated from said sender registers, means in said markerunder the control of all of said marker registers to control theconnection of said incoming trunk with said desig nated line, a localcall relay and a tandem call relay in said sender, means under thecontrol of said register operating means to operate said local callrelay following the operation of the rst of said sender registers, meansunder the control of said register operating means and said local callrelay to operate said tandem call relay following the operation of thesecond of Said sender registers, means operated in acocrdance with theregistration set up on said second register to selectively release oneof said relays, means under the control of said local call relay whileoperated to render said tandem call relay ineiective, means under thecontrol of said tandem call relay, following the release of said localcall relay, for operating said marker connecting means with a pluralityof said sender registers unoperated, and means under the control of saidtandem call relay to render said marker effective to control theextension of said incoming trunk to one of said outgoing trunks with aplurality of said marker registers unoperated.

ROBERT CAMPBELL AVERY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,541,388 Polinkowsky June 9,1925 1,595,072 CaVeily Aug. 10, 1926 2,283,395 Sti'ckler May 19, 1942FOREIGN PATENTS Number Country Date 437,591 Great Britain Oct. 22, 1935

